The invention relates generally to an electrophotographic printing, and more particularly, a cleaning blade used therein to remove particles adhering to the photoconductive member.
In the process of electrophotographic printing, a photoconductive surface is charged to a substantially uniform potential. The photoconductive surface is imagewise exposed to record an electrostatic latent image corresponding to the informational areas of an original document being reproduced. The records an electrostatic latent image on the photoconductive surface corresponding to the informational areas contained within the original document. Thereafter, a developer material is transported into contact with the electrostatic latent image. Toner particles are attracted from the carrier granules of the developer material onto the latent image. The resultant toner powder image is then transferred from the photoconductive surface to a sheet of support material and permanently affixed thereto.
This process is well known and useful for light lens copying from an original and printing applications from electronically generated or stored originals, and in ionography.
In a reproduction process of the type as described above, it is inevitable that some residual toner will remain on the photoconductive surface after the toner image has been transferred to the sheet of support material (e.g. paper). It has been found that with such a process that the forces holding some of the toner particles to the imaging surface are stronger than the transfer forces and, therefore, some of the particles remain on the surface after transfer of the toner image. In addition to the residual toner, other particles, such as paper debris (i.e. Kaolin, fibers, clay), additives and plastic, are left behind on the surface after image transfer. (Hereinafter, the term "residual particles" encompasses residual toner and other residual particles remaining after image transfer.) The residual particles adhere firmly to the surface and must be removed prior to the next printing cycle to avoid its interfering with recording a new latent image thereon.
Various methods and apparatus may be used for removing residual particles from the photoconductive imaging surface. Hereinbefore, a cleaning brush, a cleaning web, and a cleaning blade have been used. Both cleaning brushes and cleaning webs operate by wiping the surface so as to affect transfer of the residual particles from the imaging surface thereon. After prolonged usage, however, both of these types of cleaning devices become contaminated with toner and must be replaced. This requires discarding the dirty cleaning devices. In high-speed machines this practice has proven not only to be wasteful but also expensive.
The shortcomings of the brush and web made way for another now prevalent form of cleaning known and disclosed in the art--blade cleaning. Blade cleaning involves a blade, normally made of a rubberlike material (e.g. polyurethane) which is dragged or wiped across the surface to remove the residual particles from the surface. Blade cleaning is a highly desirable method, compared to other methods, for removing residual particles due to its simple, inexpensive structure. However, there are certain deficiencies in blade cleaning, which are primarily a result of the frictional sealing contact that must occur between the blade and the surface.
Dynamic friction is the force that resists relative motion between two bodies that come into contact with each other while having separate motion. This friction between the blade edge and the surface causes wearing away of the blade edge, and damages the blade's contact with the surface. For purposes of this application, volume wear (W) is proportional to the load (F) multiplied by the distance (D) traveled. Thus, W.varies.FD.varies.FVT, or introducing a factor of proportionality K, W=KFVT where K is the wear factor, V is the velocity and T is the elaspsed time. Hence, wear increases with larger values of K. Various blade lubricating materials or toner lubricant additives have been proposed to reduce friction which would thereby reduce wear. However, lubricants tend to change the operational characteristics of the printing machine undesirably. For example, a polyurethane blade with a good lubricant in the toner can ideally achieve a frictional coefficient of about 0.5, however, this rarely occurs because of the delicate balance involved in achieving the proper weight percent of lubricant in the toner. (Normal frictional coefficient values for cleaning blades that remove toner off the imaging surface range from a low of about 0.5 to a high of about 1.5). It is an object of the present invention, to consistently achieve a coefficient of friction of 0.5 or less, and thus reduce the amount of volume wear on the blade edge used for cleaning.
In addition to the problem of volume wear, blades are also subject to unpredictable failures. In normal operational configuration, with a coefficient of dynamic friction in the range of about 0.5 to about 1.5, a blade cleaning edge or tip in sealing contact with the surface is tucked slightly. The blade is not in intimate contact with the surface, but slides on toner particles and lubricant to maintain the sealing contact required for cleaning. In this configuration, the blade may flatten particles that pass under the blade and cause impaction of particles on the surface. This is called cometing because of the comet-like impressions created by the flattened particles. Also the carrier beads remaining on the surface subsequent to development may damage the blade. Another common failure is localized increases in friction between the blade and surface that cause the phenomenon of severe tucking, where the blade cleaning edge becomes tucked underneath the blade. When this occurs the cleaning blade material can fracture in the region where the severe tuck occurs and damage the blade permanently. Still another common failure occurs at start-up, when the frictional force between the blade and the surface is so high that it causes the blade to foldover on itself overstressing the blade. These types of failure require removal and replacement of the blade. Since most failures in blade cleaning systems are caused from nicking caused by tucking, it is an object of this invention, to eliminate this tucking problem, and thus, substantially improve the efficiency of blade cleaning systems.
The commonly used elastomer-type cleaning blade is a resilient material that allows stubborn residual particles to remain on the surface. This occurs because the resilient elastomeric material is unable to provide sufficient contact to create a tight seal between the cleaning blade and the surface when tuck occurs, therefore the resiliency of the elastomeric blade makes it easy for the blade to glide over the residual particles. It is an object of this invention to provide adequate rigidity to avoid the resiliency problem of the elastomer-type blade.
While it might appear that a rigid metal blade might solve the problems of rigidity and wear, in fact, the frictional contact required between the surface and blade quickly wears away the blade and any surface lubricants applied thereto. As the blade edge wears, it changes from a chiseling edge to a rounded or flattened surface which requires a high force to maintain the edge in sealing contact. While a beveled edge is useful in liquid toner applications, it is highly susceptible to damage and wear in dry toner applications. Accordingly, it is desirable to maintain the blade's square edge without wear. Additionally, wearing friction may generate toner fusing temperatures, causing toner to fuse to the blade, or the surface. Furthermore, filming on the surface can deteriorate image quality. Filming occurs either uniformly or as streaking, due to deficiencies in blade cleaning, requiring the use of a lubricant and a balancing abrasion element to prevent filming. It is an object of the present invention to provide continuous lubrication.
Various cleaning techniques have hereinbefore been used as illustrated by the following disclosures, which may be relevant to certain aspects of the present invention:
Xerox Disclosure Journal, Vol. 1; No. 4, April, 1976, page 79, "Impregnated Poromeric Material Cleaning Blade" by Paul Spencer, and Donald J. Fisher suggests the use of a porous material of non-woven fibers bound together with polyurethane which is then saturated with various materials including low friction or lubricant materials.
U.S. Pat. No. 3,936,183 to Sadamatsu describes a surface coating on the blade material.
EP-A-0 329 144 A2 discloses a silicone-modified prepolymer formed by the reaction of polyurethane prepolymer and silicone oil in a solvent.
U.S. Pat. No. 4,970,560 to Linblad discloses a metal blade that has lubricant infused into the pores of the coating only.